Controllable microwave power attenuator

ABSTRACT

A method and apparatus for controlledly attenuating the power of microwave energy passing between a source and an active load. Microwave energy is input to a first port of a circulator, whence it proceeds to a second port of the circulator, passing into a waveguide. A passive load is located in the waveguide for absorbing the incident radiation. A variable short is also connected to the waveguide, such that it embodies a variable reactance preferably in parallel with the passive load. Variation of the reactance of the variable short alters the fraction of power absorbed by the passive load, and concomitantly alters the converse fraction of the power reflected back toward the circulator. This reflected energy is directed to a third port of the circulator, at which point it may be utilized by the active load. A fourth port with an attached additional passive load may be added between the active load and the source for preventing undesirable reflections or emissions of radiation from the load back to the source.

United States Patent 91 Meddaugh Feb. 25, 1975 i CONTROLLABLE MICROWAVE POWER ATTENUATOR [75] Inventor: Gard E. Meddaugh, Mountain View, Calif.

Primary Examiner-Paul L. Gensler Attorney, Agent, or Firm-Stanley Z. Cole; John J. Morrissey LOAD [73] Assignee: Varian Associates, Palo Alto, Calif. [57] ABSTRACT A method and apparatus for controlledly attenuating [22] Flled' Sept 1973 the power of microwave energy passing between a [21] Appl. No.: 399,209 source and an active load. Microwave energy is input to a first port of a circulator, whence it proceeds to a second port of the circulator, passing into a wave- 333/17 h3i 3 guide. A passive load is located in the waveguide for [58] H i R absorbing the incident radiation. A variable short is e 0 can A 328/233 also connected to the waveguide, such that it embodies a variable reactance preferably in parallel with the [56] R f d passive load. Variation of the reactance of the variable e erences short alters the fraction of power absorbed by the pas- UNITED STATES PATENTS sive load, and concomitantly alters the converse frac- 3,l36,950 6/1964 Mackey 333/].1 X tion of the power reflected back toward the circulator. 3,202,942 8/1965 er t a]. 333/81 R X This reflected energy is directed to a third port of the 3,289,113 1l/1966 Boutelant 333/1.l Circulator at which point it may be utilized by the Z H1969 5 333/1" UX tive load. A fourth port with an attached additional 3/1969 Chm 333/81 R X passive load may be added between the active load 3,898 2/1970 Ward 333/11 3 659 233, 4/1972 Hauford 333/81 A and the source for preventing undesirable reflections 311141592 H1973 .l0l'y......::. 328/233 of emissions of radiation from the load back to source.

11 Claims, 2 Drawing Figures ASS'VE SENSOR COMMAND LOAD l DRIVE -42 '9 16 50 ACTIVE SOU RC E LOA D 2 5e 52 38 PASS IV E PATENIED FEB25|975 T0 PASSIVE LOAD 22 LOAD 40 T0 PASSIVE (COMMAND DRIVE 42 SENSOR FIG.2

CONTROLLABLE MICROWAVE POWER ATTENUATOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the field of variable attenuative control of microwave transmission.

2. Description of the Prior Art:

Many types of microwave attenuator apparatus are known. Examples of such apparatus are set forth in Microwave Circuits, Altman, D. van Nostrand Company, Inc., I964, at pages 172-178. Among these examples are loss strip, lossy wall, and matched load devices.

Prior art attenuators, while workable, have been less than totally satisfactory, particularly in high power applications. It is well known that such attenuators suffer from the disadvantages of poor frequency sensitivity and from the more significant problem that the degree of attenuation is often a function of the impedance of the load. In applications in which circulators are used as components of attenuators the circulator is the limiting component, in that its ability to handle large power loads is restricted. High power attenuators of the prior art are typically bulky and expensive. Moreover, high power attenuators generate large amounts of heat. This is particularlytrue of the lossywall variety.

Microwave power can also be controlled by varying the high-voltage direct-current (continuous or pulse) or the radio-frequency drive at the generator. However, this technique requires special electronic circuitry to assure frequency and/or power stability. It is more convenient for many critical applications to control microwave power by attenuator devices between the source and the load than by varying the voltage at the source.

Accordingly, it is a major purpose of this invention to provide a method and apparatus for providing controlled attenuation of microwave energy passing from a source to a load, wherein the degree of the attenuation is less dependent on the load impedance than in past apparatuses and methods.

It is a further object to provide a method and apparatus for controlledly attenuating microwave energy at high power levels.

It is a further. object of this invention to provide method and apparatus for isolation of the source from any reflected energy or pumped energy emanating from the load back toward the source, notwithstanding that this apparatus is used in high power applications.

It is another object to provide method and apparatus for attenuation of microwave energy in which the degree of attenuation is remotely controllable in a precise fashion.

It is a further object to provide a microwave energy attenuation apparatus that is simpler and more compact than previously provided, and in which the heat generated can be readily removed.

A further purpose is to provide apparatus and method for controlling transmission of electromagnetically radiated power with greater facility.

SUMMARY OF THE INVENTION port. The circulator propagates energy in the direction from the first port to the second'port and thence to the third port. The second, or intermediate, port is connected to a waveguide being in turn connected to a passive load. Without additional apparatus, this passive load would absorb most of the source energy which proceeds from the first to second port, very little of such energy proceeding to the active load. The waveguide, however, includes a tee to which is attached a variable short. The variable short is preferably tapped into a narrow wall of the waveguide in order to have the effect of placing a variable reactance in parallel with the passive load. When the reactance of the variable short is low, a high percentage of the power incident at the second port is reflected back out of that port and on to the active load. When the reactance, however, is high, a greater fraction of the power incident on the second port is directed to the passive load, where it is absorbed, the active load receiving little of the power input at the first port. The variable short could also be tapped into a broad wall of the waveguide, which would have the effect of placing a variable reactance in series with the passive load. However, with the variable reactance in series with the passive load, a high reactance would cause power incident at the second port to be reflected back out that port and on to the active load, and a low reactance would cause power incident at the second port to be absorbed by the passive load. It is preferable, where high-reactance conditions will be encountered, to place the variable reactance in parallel rather than in series with the passive load in order to minimize high-voltage breakdown problems that are likely to occur when power is reflected.

Additionally, a fourth port may be provided along the path of propagation within the circulator between the third and first ports. A matched passive load may be connected to the fourth port. The effect of this arrangement is to provide a means whereby any energy emanating from the port connected to the active load will be absorbed virtually in its entirety. This protects the source from unwanted reflected microwave energy from the active load, and from any energy which may be pumped out of the active load back toward the source along the propagation path of the circulator.

Apparatus, including sensing and command generation equipment coupled to drive means, is also described for controlling the position of the variable short in a precise manner from a remote location.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of the attenuation and isolation apparatus of this invention; and

FIG. 2 is a perspective view showing a circulator used as a component of the apparatus of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention of this application is clearly expressed with reference to FIG. 1. FIG. 1 shows a source 10 of Circulator 14 is of a type known in the art. lts primary characteristic is that microwave radiation input to any port will follow a path of propagation in the direction of the arrow as shown until it reaches'the next port along the path, whence the energy leaves the circulator by way of that next port. The microwave energy is intercepted by the immediately successive port, such that the energy will not be propagated to any of the remaining ports. Thus, microwave energy incident to the circulator at any given-port is capable of existing therefrom only through a unique other port.

' The microwave energy propagated by source through port 16 proceeds to port 18 from which it travels down waveguide 24, being incident on passive load 22. Passive load 22 may be any meansfor dissipating microwave energy incident thereon. Water loads known in the art are adequate for this purpose.

A portion of waveguide 24 consists of tee 26, which is connected to variable short 28. Variable short 28 is simply a waveguide having therein a slidable metal piston 30. Tee 26 and variable short 28 are suitably embodied by a variable impedance tee manufactured by Varian Associates, Palo Alto Tube Division, Palo Alto,

Cal., and designated VRS-8450. FIG. 2 provides a perspective view of a circulator 14 connected to a tee 26 in a suitable configuration according to this invention.

The positioning of slidable piston 30 in variable short 28 determines the degree to which radiation on incident to waveguide 24 through port 18 is reflected back out of waveguide 24 through port 18 without being absorbed by passive load 22. The precise relationship between the position of piston and the fraction of power incident on port 18 which is reflected is determinable only with reference to the specific dimensions l4 and port 16 is reflected back out into the body of circulator 14, from which it proceeds to port 20. Port 20 is connected by way of waveguide 32 to active load 34. Active load 34may be any type of component capable of utilizing the microwave radiative power delivered by source 10 and reflected out of port 18 without being intercepted and absorbed by passive load 22. For

example, active load 34 could be a linear accelerator of the type used in the CLlNACradiation therapy machines manufactured by Varian'Associates, Palo Alto, Cal. More generally, active load 34 could be any microwave apparatus in which it is undesirable to control the power level at the source: for example, a microwave antenna or a microwave heating applicator.

On the other hand, when piston 30 is positioned such Such isolation is accomplished by the attachment of waveguide 38 and second passive load 40 to a fourth port 36 of circulator 14. The presence of passive load I 40 attached to port 36 assures that any power in the form of microwave energy radiating for any reason from port 20 will be intercepted by port 36 and subsequently absorbed in substantially its entirety by passive load 40. Note that, in a circulator such as is used here, it is not possible for radiation incident from port 20 to enter any port except port 36. This arrangement thus prevents radiation which may at times be emitted from active load 34 and port 20 from interfering with the operation of source 10.

Provision may be made for precise adjustment of the location of piston 30 from a remote location and/or automatically. Such means may suitably include mechanical drive means 42, such as a motor and connection linkage, the output of drive means 42 being attached to piston 30. Sensing means 46 is adapted to cooperate with piston 30 such that sensing means 46 generates a signal which is a function of the position of piston 30. This signal is input to drive means 42. A command generator 44 serves to generate a command signal representative of a desired predetermined position of piston 30. The command generator 44 can have the capability of generating a plurality of discrete command signals, each signal being representative of a particular position of piston 30, where each position of piston 30 provides a particular level of microwave power transmission to the active load 34. Alternatively, the commandvsignal (or signals) can be derived from the actual radiofrequency power level at a particular location (e.g., in waveguide 32) in the system, and can be input to means for maintaining the power level at this location at a constant value in the face of unintended fluctuations in the power output of source 10. This signal is also input to drive means 42. Drive means 42 may suitably include comparison means which generates an error signal as a function of the difference between the signals from sensor 46 and command unit 44. Drive means 42 then manipulates piston 30 in order to reduce to a nullity the error signal. The specific means for accomplishing these functions are susceptible of selection by one ordinarily skilled in this art and will not be discussed in greater detail.

It is notable, however, that small differences in the position of piston 30 have very substantial effect on the fraction of power from source 10 which is ultimately transmitted to active load 34. Therefore, substantial care must be taken to assure that the control system described in the proceeding paragraph is capable of a prethat the reactance of variable short 28 is relatively high,

' a greater fraction of the energy propagated from port 16 to port 18 passes through passive load 22 to be thereby absorbed, and naturally is not reflected. Under these conditions, the fraction of power transmitted from source 10 to active load 34, is relatively low.

It is often advantageous to assure that the source 10 cise operation.

In the construction of the device in accordance with this invention, it is also necessary to assure that the frequency of the microwave energy generated by source 10 is compatible with the structure of circulator l4 and its associated components, in order that an acceptable match of frequency be achieved.

It can be seen from the above that applicant has discovered a power control for microwave radiation which is both simple and compact. This control apparatus is additionally significantly less expensive than prior art for accomplishing similar goals.

The apparatus of this invention is capable of operating at very high power levels, of the order of 5 megawatt peak power and 2.5 kilowatt'average power. Adjustment of the variable'short has been determined to yield a very flexible degree of power control, in a ratio of more than 180 to l. The relationship of power output at these power levels to the displacement of piston 30 has been determined roughly as approximately decibels per inch in the vicinity of 3 gigahertz.

It is desirable to operate the circulator and associated components filled with a dielectric gas such as SF at a pressure of approximately 30 psig. This tends to prevent undesirable arcing.

Applicant has discovered that the apparatus described hereinabove operates in such a way that the attenuation of power passing between source 10 and active load 34 is relatively unaffected by the characteristics of the impedance of active load 34. Prior attenuation and control devices have been unsatisfactory to the extent that the degree of attenuation is dependent to a relatively large extent on the characteristics of the load to which the power is delivered.

It is noted that the minimum transmissible power to the active load from source 10 is determined by the closeness of matching between the circulator port 18 and passive load 22. It is therefore desirable to choose these components to optimize this match in order to impart flexibility of control levels at low power transmission to the active load.

It is evident from the foregoing that applicant has discovered an apparatus which is compact and relatively inexpensive, yet which is capable of controlling the magnitude of microwave radiation transmitted from a source to an active load at high power levels. it is possible with this device to operate at higher power levels without arcing than have previously been attainable. Moreover, the apparatus of this invention is capable of operating at high power without generating an inordinate degree of heat.

It is to be understood that this description is illustrative only, and is not to be considered as exhaustive of the invention claimed herein. Persons of ordinary skill in the art will be able to make certain changes, modifications and alterations in the invention of this application without departing from the spirit of such invention.

What is claimed is:

ll. Apparatus for attenuating guided electromagnetic energy passing along a transmission path from a source to an active load at any frequency within the operating frequency range of said transmisson path, said apparatus comprising nonreciprocal isolation means disposed along said transmission path, said isolation means comprising a circulator having first, second and third ports,

said second port being located intermediate said first circulator throughsaid first port; second waveguide means for coupling at least a portion of said energy from said circulator to said first passive load means through said second port; continuously variable control means for controlling the amount of said energy which is absorbed by said passive load means; said continuously variable control means comprising a tee structure having a variable short, said tee structure being connected to said second waveguide means; and means for coupling the greater part of said energy which is not absorbed by said first passive load means to said active load.

2. The apparatus of claim 1 wherein said first passive load means comprises a water load.

3. The apparatus of claim 1 wherein said means for coupling said greater part of said energy which is not absorbed by said first passive load means to said active load comprises third waveguide means for coupling said greater part of said energy from said circulator to said active load through said third port.

4. The apparatus of claim 1 wherein said variable short comprises a slidable metal piston.

5. The apparatus of claim 1 wherein said second waveguide means comprises a waveguide having two broad walls and two narrow walls, said broad walls being parallel to each other, said narrow walls being parallel to each other, and said broad walls being perpendicular to said narrow walls.

6. The apparatus of claim 1 whereinsaid circulator comprises a fourth port, said fourth port being located along said transmission path downstream from said third port, said apparatus further comprising means for coupling to a second passive load means from said circulator through said fourth port that energy which is transmitted by said active load.

7. The apparatus of claim 4 further comprising means for precisely adjusting the position of said slidable piston.

8. The apparatus of claim 7 further comprising means for sensing the actual position of said slidable piston, means for generating a command signal representative of a desired position of said slidable piston, and means for automatically sliding said piston from said sensed position to said desired position in response to said command signal.

9. The apparatus of claim 5 wherein said variable short is tapped into one of said narrow walls of said second waveguide means.

10. The apparatus of claim 6 wherein said means for coupling energy transmitted by said active load to said second passive load means comprises fourth waveguide means.

11. The apparatus of claim 6 wherein said second passive load means comprises a water load. 

1. Apparatus for attenuating guided electromagnetic energy passing along a transmission path from a source to an active load at any frequency within the operating frequency range of said transmisson path, said apparatus comprising nonreciprocal isolation means disposed along said transmission path, said isolation means comprising a circulator having first, second and third ports, said second port being located intermediate said first and third ports along said transmission path; first passive load means disposed along said transmission path, said first passive load means being capable of absorbing a sufficient amount of said energy to provide a desired maximum attenuation of said energy; first waveguide means for coupling said energy from said source to said circulator through said first port; second waveguide means for coupling at least a portion of said energy from said circulator to said first passive load means through said second port; continuously variable control means for controlling the amount of said energy which is absorbed by said passive load means; said continuously variable control means comprising a tee structure having a variable short, said tee structure being connected to said second waveguide means; and means for coupling the greater part of said energy which is not absorbed by said first passive load means to said active load.
 2. The apparatus of claim 1 wherein said first passive load means comprises a water load.
 3. The apparatus of claim 1 wherein said means for coupling said greater part of said energy which is not absorbed by said first passive load means to said active load comprises third waveguide means for coupling said greater part of said energy from said circulator to said active load through said third port.
 4. The apparatus of claim 1 wherein said variable short comprises a slidable metal piston.
 5. The apparatus of claim 1 wherein said second waveguide means comprises a waveguide having two broad walls and two narrow walls, said broad walls being parallel to each other, said narrow walls being parallel to each other, and said broad walls being perpendicular to said narrow walls.
 6. The apparatus of claim 1 wherein said circulator comprises a fourth port, said fourth port being located along said transmission path downstream from said third port, said apparatus further comprising means for coupling to a second passive load means from said circulator through said fourth port that energy which is transmitted by saId active load.
 7. The apparatus of claim 4 further comprising means for precisely adjusting the position of said slidable piston.
 8. The apparatus of claim 7 further comprising means for sensing the actual position of said slidable piston, means for generating a command signal representative of a desired position of said slidable piston, and means for automatically sliding said piston from said sensed position to said desired position in response to said command signal.
 9. The apparatus of claim 5 wherein said variable short is tapped into one of said narrow walls of said second waveguide means. s
 10. The apparatus of claim 6 wherein said means for coupling energy transmitted by said active load to said second passive load means comprises fourth waveguide means.
 11. The apparatus of claim 6 wherein said second passive load means comprises a water load. 